1. Field of the Invention
The present invention relates to a structure of a rotor of a permanent magnet rotary machine in which the rotor rotates without using commutators as well as to a method of manufacturing such a rotor.
2. Description of the Background Art
Japanese Patent Application Publication No. 1997-56193 describes a rotor configured with permanent magnets used in a conventional sensorless, brushless DC motor. According to the Publication, the rotor has nonmagnetic layers made of an electrically conductive nonmagnetic material formed on a circumferential surface in an angular region (e.g., 80 to 100 degrees in terms of electrical angle) from a boundary between one north pole and an adjacent south pole of the rotor in a forward running direction and in a similar angular region from the boundary of the two poles in a reverse running direction opposite to the forward running direction.
Japanese Patent Application Publication No. 2006-109663 also describes a rotor of a conventional brushless motor. According to this Publication, a cylindrical member is fixedly fitted on the rotor, wherein an electrical or magnetic property of the cylindrical member with respect to driving coils of individual phases is varied in a circumferential direction so that inductances of the driving coils of the individual phases vary with the angle of rotation of the rotor relative to the individual driving coils.
There is a growing demand in recent years for a reduction in cost and size of a permanent magnet rotary machine (hereinafter referred to simply as the rotary machine) which is increasingly used in every kind of product. Sensorless driving technology requiring no angle sensor is one of means which can meet this demand. For a rotary machine installed in a product used under harsh environmental conditions (such as an electric power steering system), a capability to drive the rotary machine sensorlessly confers a great advantage.
Known among various forms of the sensorless driving technology which enable detection of the angular position of a rotor throughout low to high speed ranges thereof is a method based on the use of saliency of the rotor of the rotary machine. One well known example of the rotor of the rotary machine having saliency is an interior permanent magnet (IPM) rotor. The IPM rotor, however, has such problems as substantial leakage of magnetic flux within the rotor and distortion of surface flux distribution. Thus, the rotary machine employing the IPM rotor has been regarded as unsuitable for use in such a product that is required to operate with low noise and vibration, yet producing a high torque.
Under such circumstances, the aforementioned conventional rotary machines employ a surface permanent magnet (SPM) rotor which produces less leakage of magnetic flux within the rotor with a layer of a nonmagnetic or magnetic material disposed on the outer surface of the SPM rotor to impart saliency to the rotor. In the rotary machine thus structured, the angular position of the rotor is detected by measuring variations in impedances of driving coils on a stator side which occur when a voltage is applied to the rotary machine.
It is however difficult to produce sufficiently large variations in impedances in the conventional rotary machine that are needed for detecting the angular position of the rotor. Therefore, in the rotary machine of Japanese Patent Application Publication No. 1997-56193 cited above, it is necessary to apply a high-frequency voltage having a large amplitude in order to increase impedance variations. This approach however poses a problem that application of the high-frequency voltage of an increased amplitude would result in an increase in operating noise and vibration of the rotary machine.
In the rotary machine described in Japanese Patent Application Publication No. 2006-109663, on the other hand, it is necessary to increase variations in the electrical or magnetic property of the cylindrical member in order to increase impedance variations. For this purpose, the cylindrical member is structured to greatly vary in thickness so that the cylindrical member has a high eccentricity, or slits are formed in the cylindrical member having a uniform thickness. The cylindrical member thus structured may cause low-order cogging torque components to occur if the cylindrical member is made of a magnetic material, for instance, or the cylindrical member thus structured may make management of dimensions of the rotor difficult due to nonuniform mechanical gaps.